This invention relates to systems for hydraulically positioning a workpiece which includes impact loading, and more particularly, to a system which is capable of both automatically, accurately, and responsively positioning a workpiece and cushioning substantial impacting forces transmitted to the hydraulic positioning system by a workpiece.
Hydraulic drive systems are known and widely used for driving loads in a reciprocal path in which substantial force is required to accelerate the load at the beginning of the movement in each direction and substantial energy is absorbed in decelerating the load at the end of the movement in each direction. Many such systems conventionally employ a hydraulic cylinder including means for supplying hydraulic fluid under pressure to the cylinder to move a piston connected through a rod to the load.
Hydraulic drive systems are useful in a number of applications but are particularly adapted for use in sawmill drive systems. In U.S. Pat. No. 4,539,814, for example, a work operation comprising a conventional sawmill operation is described. In these types of operations, workpieces in the form of logs enter a sawmill on a conveyor and are stored on a log deck. Individual logs are then moved onto a sawmill carriage disposed along a fixed track and are moved in a reciprocal path past the saw employing suitable drive means. From a stationery starting or loading position adjacent the deck, the log and carriage are accelerated to the desired rate for sawing, are then moved at a substantial uniform rate as the log traverses the saw at a sawing station to have a slab or board removed therefrom. Toward the end of the sawing operation, the log and carriage are decelerated and ultimately stopped after the log has moved completely past the saw. The log and carriage are then driven in opposite directions and returned to a starting position where it is again brought to a stop after the log has again cleared the sawing station. The log is indexed laterally on the carriage to enable a second board to be removed and the process repeated until an optimum amount of boards are cut from a given log. Because of the heavy weight of the carriage and logs, substantial energy is required to accelerate and decelerate the carriage at the beginning and end, respectively, of its movement in each direction. Also, for economical reasons, it is desirable to repeat the cutting procedure as quickly as possible to maximize the number of board feet produced by a given sawmill system. Therefore, extremely large forces are involved in accelerating and braking the carriage in order to maximize the sawmill operations.
In U.S. Pat. No. 2,613,703, for example, a volumetric hydraulic system, including means for accumulating a preselected volume of pressure fluid and for discharging same into an appropriate delivery conduit is provided. This hydraulic system includes means for accumulating a preselected volume of pressure fluid and for injecting same into a hydraulic actuator for displacing the reciprocal member thereof through an increment of movement proportional to such preselected volume. In this way, the position of a set of one or more moveable knees in a sawmill is precisely controlled.
In U.S. Pat. No. 3,361,034, a load positioning device is provided, having infinite settings throughout its operating range and a predetermined set position which is maintained from the same direction. The device allows a load to be automatically moved in a desired direction, from an initial setpoint, upon selection of a new setpoint. All of this is accomplished employing a hydraulic positioning cylinder.
A sawmill setworks is provided in U.S. Pat. No. 3,068,914 having hydraulic actuating means in which accurate setting is obtained by injecting a pre-determined quantity of hydraulic fluid into the hydraulic setworks mechanism.
None of the above described prior art systems include means for cushioning the impact during impact loading of a workpiece or during hydraulic positioning of a workplace, respectively, but merely deal with the positioning aspects per se.
In U.S. Pat. No. 4,539,814, a hydraulic drive system for a carriage drive circuit is provided in which accumulators are used for reciprocatingly driving a heavy load. Thus, averaging accumulators and braking accumulators are employed for driving a sawmill carriage. Deceleration of the moving carriage is accomplished by an accumulator which is set to operate at or above the main system operating pressure. Thus, this system is designed to decelerate and cushion a moving cylinder piston connected to a high inertia load.
U.S. Pat. No. 4,335,993, and U.S. Pat. No. 4,409,792, relate to hydraulic positioning apparatus for precisely and rapidly positioning objects of the type likely to exert high impact or shock loads on the positioning apparatus. In particular, these patents relate to improvements in hydraulic positioning of the automatic type in which a controller responsive to the position of the apparatus automatically controls its movement so as to obtain precise positioning. The system comprises a hydraulic positioning cylinder which is constantly balanced against system pressure at the source of pressurized hydraulic fluid through a one-way check valve which opens fully instantaneously and allows immediate relief of over-pressurized fluid when the pressure of the fluid in the motor exceeds the operating hydraulic pressure of the system. In other words, over pressurized fluid resulting from an impact load is routed through a one-way check valve back to the main supply accumulator. This allows absorption of a gross portion of the impact energy, but its real effectiveness is highly dependent on the main system pressure and the state of charge or discharge of the main supply accumulator. Since the pressure level in this accumulator is at maximum operating pressure, impact pressure will only be absorbed when it exceeds the operating pressure of the system. However, for maximum fast reaction and effectiveness, and to instantaneous cushion impact loads, one must be able to initiate the cushioning process at a pressure substantially less than the maximum system supply pressure.
U.S. Pat. No. 4,344,609 is directed to a knee carriage cushioning device using a gas-filled pneumatic device for resiliently absorbing force from an object moving against a resilient positioning apparatus. Pneumatic gas pressure must be set at a high enough level so that it does not interfere with normal working loads. Therefore, as in the case of U.S. Pat. No. 4,335,993 and U.S. Pat. No. 4,409,792, high-impact pressures must result before impact-absorbing action will occur.
In U.S. Pat. No. 4,779,513, a pneumatic positioning device is described which includes a pneumatic cylinder, accommodating piston movement in the cylinder in to supply and discharge of compressed air. As described with respect to U.S. Pat. No. 4,344,609, for pneumatic gas-pressure devices, high operating pressures must be set and overcome in this system in order to lock out the circuit for normal operation.
Accordingly, a need exists for a hydraulic positioning system which is able to provide for maximum fast reaction to cushion instantaneous impact loads both at pressures above the operating pressures on the one hand, and at pressures below operating pressure on the other hand.